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Different generations of solar cells

Solar cells are usually divided into three main categories called generations. The first generation contains solar cells that are relatively expensive to produce,
and have a low efficiency. The second generation contains types of solar cells that have an even lower efficiency, but are much cheaper to produce, such that
the cost per watt is lower than in first generation cells. The term third generation is used about cells that are very efficient. Most technologies
in this generation is not yet commercial, but there is a lot of research going on in this area. The goal is to make third generation solar cells cheap to produce.

First Generation Solar Cells:

The first generation includes cells consisting of Silicon or Germanium that are doped with Phosphorus and Boron in a pn-junction. This generation is dominating the
commercial market. Silicon cells have a quite high efficiency, but very pure silicon is needed, and due to the energy-requiring process, the price is high compared
to the power output.

This is the generation that we focus on in the advanced section of this site.

Multicrystalline silicon solar cell.

Second Generation Solar Cells:

Amorphous Silicon Cells

In Amorphous Silicon Cells, hydrogen is introduced to the silicon to make it possible to dope the silicon with boron and phosphorus. The cells are built up
in this sequence from bottom to top: metal base contact, n-layer, intrinsic layer, p-layer, transparent contact, glass substrate. These cells experience a drop in
efficiency when they are exposed to sunlight, and this effect is created in the intrinsic layer. The effect can be reduced by, instead of one layer, using several
thinner layers.

Calculator with amorphous silicon solar cell.

Polycrystalline silicon on low cost substrate

These cells use antireflection layers to capture lightwaves with wavelengths several times greater than the thickness of the cell itself. This can be done by using
a material with a textured surface both in front and back of the cell, rather then a flat surface. This causes the light to change directions and be reflected, and thus
travels a greater distance within the cell then the cell thickness.

Polycrystalline silicon solar cell

Copper Indium diSelenide (CIS) Cells

Copper Indium Diselenide consist of CuInSe2. This material is one of the best light absorber known, and about 99% of the light is absorbed before reaching 1 µm into the
material. There have been made homojunctions of CIS, but a heterojunction with cadmium sulfide(CdS) has
been found to be more stable and efficient.

Flexible Copper Indium Gallium diSelenide solar cells

Cadmium Telluride Cells

These cells are made from a heterojunction with cadmium sulfide, just like the copper indium diselenide. Cadmium telluride cells also have an ideal bandgap(1.44eV).

Third Generation Solar Cells:

There are several techologies in this generation. One of them is Quantum Dot(QD) Solar Cells. These are built up of a semiconductor(silicon) coated with a very thin layer of quantum dots. Quantum dots is just a fancy name of crystals in the
size range typically a few nanometers in diameter. These crystals are mixed into a solution and placed on a piece of silicon which is rotated really fast. The crystals
are then spread out due to the centrifugal force.
The reason these quantum dots are given so much attention is that normally one photon will excite one electron creating one electron-hole pair. The energy loss is the
original energy of the photon minus the energy needed to excite the electron(also called the band gap) However, when a photon hits a quantum dot made of the same material,
there may be several electron-hole pairs created, typically 2-3, but 7 has been observed.

Another way to increase the efficiency is to use several layers solar cells with different band gaps in a stack. Each layer will utilise light with different wavelengths,
and in this way we can get cells with a higher efficiency.